Process for electrolytic deposition of metallic chromium



Patented June 21, 1932 umreo STATES PATENT OFFICE DWIGHT T. EWING, OF EAST IANSING MIGHIGAN No Drawing.

The invention relates to the electrodeposition of chromium and more particularly to the preparation of an electrolyte from which the chromium may be deposited on a cathode to form a bright, lustrous finish suitable for commercial requirements.

In the present state of the art it is known that electrolytes containing chromic acid alone are not satisfactory for commercial chromium plating but I have discovered that by the addition of certain other substances to chromic acid solutions it is possible to so change the character of the electrolyte that a very high grade chromium plate may be obtained therefrom. Amongthe various substances which when added to the chromiumcarrying electrolyte will produce a good quality of plate are compounds of metals which readily exist in two different states of valence and include the following: mercury, mol denum, cerium, manganese and possibly others. These substances in some instances may be in the form of metallic salts while in other instances they may be the oxides For example, a suitable electrolyte may be prepared by adding to a chromic .acid

solution twenty grams of cerium carbonate afor'each liter of solution and stirring as long as efi'ervescence continues and until the cerium is completely in solution. During this process a chemical change has taken place and the carbonate radical has been decomposed while at the same time an equivalent amount of the chromic acid has been'neutralized. This neutralization of a portion of the chromic acid improves the quality of the plating bath but is not the only reason for adding the cerium carbonate. The cerium ion acts as an oxidizing agent as will be explained later. The solution thus prepared is now entirely satisfactory for use in an electrolytic process where the article to be plated is immersed in the solution as a cathode and direct current is passed thereto from a suitable anode. Preferably the anode employed is of such a character as to be not decomposed and it may sometimes be the walls of the plating tank.

' It is also entirely satisfactory to add mercury oxide to a 30% solution of chromic acid Application filed July 30,

1926. Serial 126,094.

in which case a somewhat larger amount of material is used for each liter of solution. For example, the addition of 40 grams of mercuric oxide for each liter of solution, and stirring the solution vi orously will render it entirely satisfactory or the deposition of chromium. The mercury may also be introduced in other forms such as soluble mercurous or mercuric salts. I have also found that materials other than cerium andmercury also impart the same characteristics to the electrolyte. Good deposits of chromium plate have been obtained by the addition of molybdenum compounds, or manganese compounds.

While it is not necessary to know the theory underlying the addition of these various compounds to a chromic acid electrolyte in order to practically apply my invention, I have nevertheless noted that the addition of compounds of certain meta-ls which readily occur in two valent states, imparts to a solution ofchromic acid those essential characteristics which enable a good chromium deposit to be obtained. It will be noted also that the metals which. I have used in my experiments are all capable of forming compounds of the type known as oxidizing and reducing substances. The reaction proceeds very smoothly under suit able conditions of temperature and current density and is readily followed by noting the change in color of the solution. While the original solution is red the color of the bath upon continued operation darkens until the color appears almost black. Just what chemical changes have taken place in the chromic acid in solution when the above mentioned substances are added and a current is passed, are not completely known. At least a sufiicient amount of the original chromium present in the chromate'radical has been changed to a lower. state of oxidation; The essential reaction has taken place,

namely, the decomposition of the chromate 9 radical and resultant formation of chromium in solution and in a state of oxidationwhich is necessary for procuring deposits of metallic, chromium suitable for commercial uses. The amounts of the oxidizing-reducing substances which I have added is suflicient to prevent the chromicchromium. from returning to the chromate form.

According to my present understanding of the action of my improved chromium plating baths the presence of the ions of mercury or cerium in a chromic acid electrolyte causes the chromium in the solution to be maintained in its hexavalent state and effectively prevents the formation of any substantial amount of trivalent chromium in the bath during electrolysis. One of the advantages incident to the freedom of the bath from trivalent chromium is that the conductivity is better and the bath is therefore more eflicient. Such solutions as depend upon the addition of chromic compounds (chromio sulphate, chromic chromate, chromic hydroxide, etc.) have in general greater resistivity to the passage of the electric current. My electrolytes are made from chromium compounds where the chromium exists only in the hexavalent state, that. is, they are made from commercial chromic acid free from any substantial amount of trivalent chromium and no additional chromic salts or hydroxides are placed in the bath. v

In order to secure the best results it is necessary to control the factors of temperature and current density within certain limits and as an example of suitable conditlons 1t 18 satisfactory to maintain a temperature of 30 to 40 C. while employing a current density of 6 to 20 amperes per square decimeter. It'is to be understood, however, that these are not the only conditions under which the process may be carried out with the improved electrolytes forming the subject matter of my invention.

()ne of the important advantages of my process is that a higher current elliciency is obtained than in processes heretofore developed for the depositing of chromium. Thus, for a given amount of current passing through the electrolyte my process results in the liberation of more chromium and less hydrogen at the cathode than with other processes. In other words the ratio of chromium to hydrogen is higher in my process.

It is also to be understood that my invention is not limited to the specific quantities of mercury and cerium given in the preceding paragraphs for it is only necessary to use that amount which will render the chromium-carrying electrolyte in such condition that a satisfactory deposit is obtained.

For example where a nercury compound is used as the additional agent, I have obtained gooddeposits of chromium "from a solution where only g. of mercuric oxide has been added to each liter of solution and have also obtained satisfactory results with as much as 40 g. perliter. However, it is probable that amounts outside of this range can also be used under certain conditions. It is further to be understood that in the examples given the mercuric oxide has been added to a chromium carrying electrolyte which without any additional substance could not be used for production of a satisfactory chromium plate.

Still another advantage of my process resides in the fact that the addition. of the above enumerated substances, particularly the mercury compound, aids materially in securing a bright finish. Thus a bath which is capable of being used for depositing chromium under certain conditions but which results in a dull finish on the plated object can be so greatly improved by the addition of a mercury compound, in accordance with my invention, that under the same conditions a much brighter finish is obtained.

What I claimas my invention is:

1. The process of electrodeposition of chromium which consists in electrolyzing an electrolyte consisting principally of a solution of chromic acid free from. trivalent chromium and in maintaining the chromium in solution in the hexavalent state substantially free from trivalent chromium by electro lyzing the solution in the presence o1 a metallic ion having strong oxidizing properties. V i

2. An electrolyte for electrodeposition of chromium comprising a. solution of chromic acid substantially free from travalent chromium and a compound of a metal having two valent states and having oxidizing properties such as to be capable of maintaining substantially all of the chromium in the hexavalent state during electrolysis.

3. An electrolyte for electrodeposition of chromium comprising a solution of commercial chromic acid substantially free from trivalent chromium and a small amount of a compound of a metal having at least two valent states and being capable of oxidizing trivalent chromium to the hexavalent state during electrolysis.

4. An electrolyte for electrodeposit-ion of chrom um comprising a solution of commercial chromic acid substantially free from trivalent chromium and a small amount of a mercury compound for maintaining the electrolyite substantially free from trivalent chromium during electrolysis.

5- An electrolyte for electrodeposition of chromium comprising a solution of chromicacid substantially free from trivalent chromium and a compound of a metal capable of maintaining substantially all of the chromium in the hexavalent state during electrolysis.

In testimony whereof I aflix my signature.

DWIGHT T. EWING. 

